The present invention relates to a carrier detection unit used in a radio data communication system of a spread spectrum system using a plurality of spread codes and a carrier detection method. More particularly, the present invention relates to the carrier detection unit and carrier detection method allowing for carrier detection both at preamble mode and data mode.
Packet transmission or the like has been conventionally employing a radio data communication system of a spread spectrum system using a plurality of spread codes (JP-A 12267/1995).
A reception side of a communication unit in the aforementioned radio data communication system is provided with a carrier detection unit for detecting a carrier of the received wave. The carrier is detected only at preamble mode for synchronization. Conventionally the carrier detection is not executed at data mode.
The radio data communication system of spread spectrum system using a plurality of spread codes is described at preamble mode and data mode, respectively.
Preamble mode!
At a transmission side, 1 bit serial data for synchronization are information modulated through QPSK or the like. Each bit of the above information modulated serial data is a signal in which 1 symbol contains a predetermined number of chips. After the information modulation, a predetermined one kind of spread code is multiplied by each symbol with respect to each chip for spread modulation. The spread modulated signal is transmitted by a transmitter as a spectrum spread signal.
At a reception side, the transmitted spectrum spread signal is received, A/D converted and held by each symbol. The spread signal identical to that used in the transmission side is multiplied by each symbol per chip for de-spreading.
Detecting the carrier during reception causes auto correlation at a constant level by each symbol resulted from de-spreading. Therefore the presence/absence of the carrier can be detected by detecting the auto correlation. In order to establish synchronization, stable carrier detection is inevitable. Then an absolute value (hereinafter referred to as a correlation value) of an amplitude of the auto correlation obtained from each symbol is integrated. When the integrated value reaches a predetermined threshold value or more within a predetermined synchronization integration period, it is judged as presence of carrier, i.e., synchronization has been established. When the integrated value is less than the threshold value, it is judged as absence of carrier, i.e., synchronization has not been established yet. As for the absence of carrier, the correlation value stored in the synchronous integration section is cleared and set to "0" for re-establishing synchronization by executing synchronization integration again. Both the synchronization integration period and the threshold value can be set to any values depending on a balance between carrier detection time and the circuit size. An initial value "0" is preset in the synchronous integration section.
Data mode!
Upon synchronization established at preamble mode, the communication mode is switched to data mode where information signals are transmitted/received.
At the transmission side, the information signal containing 1 bit serial data is converted into n bit parallel data. An m bit string of the n bit parallel data is modulated through QPSK or the like. At this time, a spread code having a predetermined code length corresponding to each of the bit pattern of the (n-m) bit string that has not been information modulated is generated. The spread code has maximum of 2.sup.(n-m) variations for corresponding to each bit pattern of the (n-m) bit string one by one.
The generated spread code is multiplied by each bit (symbol) of the corresponding m bit string that has been information modulated by each chip for spread modulation. The spread modulated m bit string is transmitted as a spectrum spread signal from a transmitter.
At a reception side, the transmitted spectrum spread signal is received and A/D converted for holding in the input order by each symbol. The held signal is parallel transferred and latched at a synchronization point. The signal is then inverse spread with the spread code identical to that used for spreading.
At data mode, a spread code selected from 2.sup.(n-m) variations is used depending on the bit pattern of the (n-m) bit string during transmission. Therefore at the reception side, it is required to specify as to which spread code has been used during transmission. In spectrum spread modulation, the correlation value becomes maximum through de-spreading with the spread code identical to that used for spreading. So the spread code generating the maximum correlation value is obtained. More specifically, all spread codes of 2.sup.(n-m) variations are inverse spread by each symbol. Then the spread code generating the maximum correlation value of obtained correlation values is selected.
Based on the spread code generating the maximum correlation value, the spread code used for spreading during transmission is, thus defined. The bit pattern of (n-m) bit string corresponding to the above obtained spread code can be specified. The maximum correlation value is demodulated so as to obtain an m bit string. Using the (n-m) bit string and m bit string, the n bit parallel data that have been transmitted are reproduced.
In the conventional carrier detection unit, the carrier detection is executed only at preamble mode and not executed at data mode. However it is possible for the conventional carrier detection unit to execute carrier detection at data mode.
The carrier detection at preamble mode and data mode in the conventional carrier detection unit is described below referring to the drawings.
FIG. 4 is a block diagram showing a conventional carrier detection unit provided at the reception side.
FIG. 2 is a wave chart showing the reception level at preamble mode. In FIG. 4, a reference numeral 1 is a correlator for detecting auto correlation of a spectrum spread signal; 2 is an amplitude detection section for detecting an absolute value of the amplitude output from the correlator 1; 3 is a synchronous integration section for integrating the absolute value output from the amplitude detection section 2 with respect to each symbol; and 4 is a carrier detection section that judges as "presence of carrier" when the integrated value output from the synchronous integration section 3 is a predetermined threshold value of more and as "absence of carrier" when the integrated value is less than the predetermined value. In FIG. 2, 21 is a wave showing the amplitude of the auto correlation output from the correlator 1; 22 is a wave showing the absolute value of the wave 21 output from the amplitude detection section 2; and 23 is a wave showing the integrated value of the wave 22 output from the synchronous integration section 3.
The operation of the above constructed conventional art is described with respect to the preamble mode and the data mode.
Preamble mode!
A received spectrum spread signal is A/D converted and supplied to the correlator 1. The correlator 1 inverse spreads the signal using the spread code that has been used during transmission so that an auto correlation amplitude is obtained (wave 21). The obtained amplitude is supplied to the amplitude detection section 2 where the absolute value of the amplitude is obtained (wave 22). The absolute value is supplied to the synchronous integration section 3 for synchronization integration (integration of the input signals). Then the obtained value is integrated by each unit of a symbol in a shift resistor (not shown) provided with the synchronous integration section 3 (wave 23). The obtained integrated value is always compared with a predetermined threshold value in the carrier detection section. When the integrated value exceeds the threshold value within a predetermined synchronization integration period, it is judged as "presence of carrier". When the integrated value does not exceed the threshold value, it is judged as "absence of carrier". As for the absence of carrier, the integrated value of the shift resistor is cleared and reset to 0. The integration is executed again and the aforementioned procedure is repeatedly executed until the synchronization is established.
Data mode!
FIG. 5 is a wave chart showing the reception level during carrier detection at data mode. In FIG. 5, 51 is a wave showing the auto correlation amplitude output from the correlator 1; 52 is a wave showing the absolute value output from the amplitude detection section 2; and 53 is a wave showing the integrated value of the wave 52 output from the synchronous integration section 3.
At data mode, the timing for auto correlation by each symbol may be shifted by a unit of a chip because the spread code is changed depending on the pattern of the (n-m) bit string (See waves 51 and 52). As the integration is executed leaving the timing shifted, the integrated value hardly increases (See wave 53). As a result, the integrated value cannot reach the threshold value within the synchronization integration period. Then it is misjudged as "absence of carrier" in spite of presence of carrier, thus terminating reception forcibly. In order to prevent the aforementioned misjudgment, the synchronization integration period has to be set to a relatively long time so that the integrated value reaches the threshold value. Otherwise the threshold value has to be set to a smaller value.
As aforementioned, in the conventional art, the synchronization integration period has to be set to a longer time at data mode for carrier detection compared with preamble mode. For this the control circuit size or the circuit size of the correlator for prolonging such period have to be increased.
In case the threshold value is set to a smaller value, the carrier cannot be detected accurately because of adverse effect of noise.